The present invention generally pertains to slow scan television systems and is particularly directed to systems for translating color slow scan video signals.
Slow scan television systems utilize slow raster scan techniques in order to enable video signal image information to be transmitted over narrow bandwidth audio communication channels, such as the telephone lines. Such systems are described in two articles by Steber entitled "SSTV to Fast Scan Converter" appearing in the March 1975 issue of QST at pages 33-40, and "Slow-Scan to Fast-Scan TV Converter" appearing in the May 1975 issue of QST at pages 28-36 and 46. A slow scan television scan converter is described in U.S. Pat. No. 4,057,836 to Munsey. In Munsey's system, either fast scan video signals from standard closed-circuit television cameras or received slow scan video signals are stored a frame at a time in a digital memory. The stored slow scan video signal then is read out of the memory and transmitted at a slow scan video signal transmission rate over a narrow frequency band, using well known frequency modulation techniques. One complete frame can be transmitted on the order of every 8 to 32 seconds depending on the desired quality of the received image. The received slow scan video signal is then stored in the memory of the receiver system, from which it is read out at a fast scan rate for generating a display on a standard closed circuit television monitor.
Due to the restrictions placed on the transmission of slow scan video signals by the narrow bandwidth of the transmission channel, only a few techniques have been employed for the transmission of a full color slow scan video signal that includes three complementary color component slow scan video signals. One such technique is the transmission of sequential frames of red, green and blue component signals and storing each frame in a separate slow-to-fast scan converter and reading out all three signals simultaneously at the fast scan rate for generating a display on a color closed-circuit television monitor. The major drawback of this technique is the amount of transmission time required to send and receive the three complementary color component slow scan video signals necessary for generating a display of one composite color image. The transmission time is three times as long as for monochrome slow scan video signal transmission or about 24 to 96 seconds depending on the desired received image resolution. An additional drawback of this technique is that the composite three color image is not available for viewing until reception of the last of the three complementary color component signals.
Another prior art technique is the interleaving of the red, green and blue component signals in a line interlace fashion, resulting in a system that still takes three times longer than its monochrome counterpart but which allows the viewer to inspect the image as a complete color entity as it is received.
Still another prior art technique is to transmit a frame of luminance information at the monochrome slow scan video signal transmission rate followed by the transmission of two color difference information frames at twice the monochrome rate, but at a lower level of resolution. The lines of video information are interlaced to give the appearance of a continuous color transmission. This technique still takes twice as long as the transmission of a monochrome slow scan video signal.
A technique that would result in the complete transmission of a color slow scan video signal in a time equivalent to that required for transmission of monochrome slow scan video signal has been suggested. According to this technique, an amplitude modulated chrominance information subcarrier in a frequency band below the band used for frequency modulating the luminance information signal is quadrature modulated with two color difference information signals. One problem with this technique is that the chrominance information subcarrier would necessarily be less immune to noise due to its amplitude modulated nature and would not hold up well in the noisy channels often encountered on telephone lines and through radio frequency transmissions. Additionally, amplitude modulated systems do not tolerate magnetic tape dropouts, which are often encountered when slow scan video signals are stored for transmission or playback on audio tape equipment.
Yet another prior art technique for the simultaneous transmission of luminance and chrominance information is to amplitude modulate the already frequency modulated luminance subcarrier with the chrominance information much like that proposed for commercial AM radio stereo broadcasts. The defects in this technique as applied to slow scan television are similar to those of the previously discussed technique. In addition the opportunity for cross talk is significant. The fact that the amplitude modulated carrier frequency is shifting within the band in accordance with the frequency modulation together with the fact that the transmission channel is seldom linear in the band of interest results in an amplitude distortion generated by the frequency modulated component.